Plan and interpret a controlled diffusion investigation
Choose variables, repeat measurements and calculate a stated proxy for diffusion rate.
Section B3.1 · Diffusion
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall the four diffusion factors and simple averages.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why repeat diffusion measurements?
What you will learn
- Choose variables, repeat measurements and calculate a stated proxy for diffusion rate.
- Apply the idea to a supplied biological example or investigation.
Design one question at a time
A diffusion investigation should identify the independent variable, a measured dependent variable and relevant controls. A classroom model might compare the time for a colour front to travel a fixed distance in identical gel samples at different temperatures. Teacher-approved materials and handling are required for practical work.
Keep gel composition, dimensions, starting dye concentration, exposed area, measurement distance and measurement method constant. Repeat measurements at each temperature and use a mean. A control comparison and repeats address different issues: fair conditions limit confounding, whereas repeats help reveal variation.
Interpret the measured quantity honestly
For a fixed measured distance, distance divided by time is a useful average front-speed proxy. It is not automatically the molecular diffusion coefficient or the total mass transferred per second. Label the quantity and its units, and compare like measurements.
Use a table with units and a plot appropriate to the independent variable. A colour endpoint can be subjective, so use a defined criterion. Model gels are not living tissues; an investigation can support a diffusion trend without certifying laboratory competence or proving every cell behaves identically.
Sources for this lesson
Pause and explain
Times for the same endpoint are 4, 5 and 6 minutes. What is the mean time?
Worked example
A colour front travels 12 mm in 6 minutes. Calculate the average front-speed proxy.
Show the worked solution
- Use measured distance ÷ time.
- 12 ÷ 6 = 2.
- Attach mm/min and label the proxy.
Answer 2 mm/min average front speed.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A front travels 18 mm in 9 minutes. Calculate its average speed proxy.
Give me a hint
Divide distance by elapsed time.
Compare my reasoning
- 18 ÷ 9 = 2.
- Use mm/min.
2 mm/min.
Look for these in your work
- Name the measured proxy.
Endpoint times are 5, 6 and 7 minutes for one condition. Find their mean.
Give me a hint
Add and divide by three.
Compare my reasoning
- Total = 18 minutes.
- 18 ÷ 3 = 6 minutes.
6 minutes.
Look for these in your work
- Use all repeats.
A gel experiment uses a thinner sample only at the higher temperature. Explain the problem and repair the design.
Give me a hint
Distance changed with temperature.
Compare my reasoning
- The shorter path may accelerate the endpoint.
- Use equal thickness for every temperature.
- Repeat at each condition using the same endpoint.
Thickness is a confound; match thickness and repeat the comparison.
Look for these in your work
- Identify distance as the confound.
Common mistakes
- A model result is not automatic evidence of whole-organism behaviour.
- Distance/time is a labelled proxy, not a diffusion coefficient.
What can you explain now?
Times for the same endpoint are 4, 5 and 6 minutes. What is the mean time?
Compare with the explanation
5 minutes.
(4 + 5 + 6) ÷ 3 = 5.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Design a fair test for one of area, temperature, gradient or distance.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.